Form filters are used to prevent massive contrasts in the image signal due to radiation protection and image quality. A form filter is a semi-transparent filter which absorbs more x-ray radiation in the beam path upstream of relatively x-ray transparent body areas like for instance the lungs and/or object parts but absorbs less x-ray radiation in the beam path upstream of body areas which themselves absorb relatively large quantities of the x-ray radiation, for instance bones. If applicable, a region of the form filter is also used to attenuate direct radiation, in other words such x-rays which do not penetrate the image object but instead run adjacent thereto at the edge for instance and reach the x-ray radiation detector.
Conventionally form filters were previously positioned purely manually. Alternatively, positioning with the aid of presaved positions is conventional. A manual re-positioning of the semitransparent filters is necessary particularly when recording parameters are changed, e.g. the acceleration voltage with respect to the x-ray radiation source or the image enlargement etc. Such a re-positioning is frequently not implemented due to lack of time. If applicable, a low-grade image quality is achieved as a result and the image object obtains an excessive x-ray radiation dose.
The technique of recording an x-ray image of the image object, detecting contrast-rich edges and positioning the filter along these edges also exists in the prior art.
This is disadvantageous in that a relatively high x-ray radiation dose is applied to the image object solely for the test image.